Shift work linked to risk of heart attack, stroke, new study suggests

Shift work — especially working nights — increases the risk of heart attack and stroke, according to a Canadian-led study that is being billed as the largest-ever of its kind.

About a third of Canada's full-time labour force does shift work — working evening and nights, rotating shifts, split shifts and any other schedule that isn't nine-to-five.

Until now, the data on whether shift work heightens heart attack and stroke risk has been controversial, with some studies showing an increased risk, and others no association whatsoever.

For their new study, the international research team pooled results from 34 studies involving more than two million people. They found that shift work was associated with a 23 per cent increase in the risk of a heart attack, a five per cent increase in the risk of stroke and a 24 per cent increase in the risk of unstable angina, coronary artery disease and other "coronary events."

Night work was associated with the sharpest increase in risk — 41 per cent — for major vascular problems.

When extrapolated to the Canadian population, "about one in 14 heart attacks and just under one in 60 strokes are directly related to shift work," said senior investigator Dr. Daniel Hackam, an associate professor and clinical pharmacologist at Western University in London, Ont.

The findings held after other factors that could skew the results were taken into account, such as smoking — meaning that other unhealthy behaviours couldn't explain the association, the authors said.

"One in three adult Canadians who are employed full-time are shift workers, so it's a highly prevalent employment condition," Hackam said. "And we're going towards a 24/7 society — there's even some talks of banks being open 24 hours a day.

"We don't think shift work is going to go away."

The team reviewed studies published from the 1960s to 2012 involving 2,011,935 people. They found that heart attacks and ischemic strokes — where a blood clot lodges in a vessel in the brain, squeezing off blood flow — were more common among shift workers than other people.

Published in the British Medical Journal, the study is the "largest synthesis of shift work and vascular risk reported to date," according to the authors.

They haven't proved cause-and-effect, merely an association.

The risk for any one person is modest but, given shift work is so widespread, the overall risks for the population are high, Hackam said.

Just how the erratic schedules of shift work might heighten heart attack and stroke risk isn't clear, but several mechanisms might be at play. "We may just not be hardwired to be working in the night," Hackam said.

Shift work disrupts circadian rhythm — the body's 24-hour sleep-wake cycle. Blood pressure, heart rate and even cholesterol levels all return to low levels during sleep while the body rests.

"That's just not the case in someone who is awake and engaged in work," Hackam said. "Their blood pressures are going to be higher, their heart rates are going to be higher, their cholesterol and (blood) clotting factors — they're going to be exposed to cardiovascular risk while the rest of us are sleeping and repairing our bodies." According to the authors, even a single overnight shift is enough to increase blood pressure, and insomnia itself is a risk factor for heart attack.

Our bodies also aren't genetically adapted to eating in the middle of the night. "Most of us are fasting while we sleep and not accumulating fat and carbohydrates at night," Hackam explains. Night eating can lead to spikes in blood glucose and insulin, which are toxic to the arteries and increase the risk of diabetes.

Another factor may be exposure to electric light at night — a "relatively recent phenomenon," Hackam said. Lab experiments have shown that within seconds of exposure to bright light at night, melatonin — a hormone that lowers blood pressure — drops dramatically, essentially to undetectable levels, Hackam said.

Shift work also plays havoc with work-life balance. "People often miss time with their families, so there are psychological consequences of that," he said.

Employers should consider workplace screening programs for cardiovascular risk factors, and workers should be "vigilant" about having their blood pressure, cholesterol, waist circumference and blood sugars checked regularly, Hackam said.

"They should try to bring food from home, take breaks if possible and get sufficient amounts of sleep when they're not working. It's essential the human body is rested a minimum of eight to nine hours."

The study's major strength is its sheer size, said Dr. Andrews Wielgosz, a professor of medicine at the University of Ottawa and a spokesman for the Heart and Stroke Foundation. "It's a powerful observation," he said. "But at the end of the day, it's still an observation, which doesn't explain what's going on."

It could be less about the shift work itself, he said, and more about the types of people who engage in it. Shift workers are more likely to have a lower socioeconomic status than day workers. They also have higher rates of high blood pressure and metabolic syndrome — a cluster of conditions that increase the risk of heart attack and strokes.

According to Statistics Canada, in 2005, more than four million workers aged 19 to 64 in 2005 worked something other than a regular day shift.

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